After installing the latest update to my Apple TV, I was having a look to see what had changed. There are a couple of things that have been posted on the net; for instance, the new Remote app for the iPhone/iPod touch, the new UI that sorts shows with the oldest first rather than the newest, and the ability to display unwatched episodes only.

However, one feature that I have not seen documented is that when you have a TV Show or Movie highlighted, if you press and hold the center button on your remote for a second or so, a menu will appear allowing you to mark the item as Watched.